1. Technical Field
This invention relates to selective deposition, more particularly to a method of selectively depositing a metal layer on an existing metallurgy pattern, utilizing liftoff techniques.
2. Background Art
The multi-layer ceramic (MLC) technology for producing substrates for integrated circuit semiconductor package assemblies is relatively well known. Such substrates are produced by preparing a slurry of a suitable particulate ceramic, a resin binder material, a solvent for the resin binder, and generally a plasticizing agent, doctor blading the slurry on a base and subsequently drying to form thin flexible sheets, commonly termed ceramic green sheets. These sheets are punched to form via holes, and the via holes filled with a conductive paste and also formed into lines which will ultimately form the internal circuitry. The punched and printed green sheets are assembled into a laminated substrate often consisting of from 15 to 30 sheets and the unit sintered. The resultant substrate is capable of mounting many devices which are interconnected by the internal circuitry. External contact is made by plurality of I/O pins on the opposite side. It is desirable that the multi-layer ceramic substrate be formed with the lines and via holes conforming to very small dimensions. Such microminiaturization is desirable in order that the package be compatible with the integrated circuit device chips mounted thereon. The substrate must be provided on the top surface with many very small pads which are capable of making electrical contact with the corresponding, closely spaced device terminals. In order to more efficiently use the modern integrated circuit technology, as many as possible integrated circuit devices are supported on and interconnected within the same substrate. This arrangement keeps the distance between interconnected devices small and thereby minimizes the time it takes for electrical signals to travel from interrelated devices. Further this reduces the number of electrical connections which must be made and thereby reduces the cost of the package and increases the reliability. The end result is a highly complex multi-layer ceramic package with a lot of small internal printed circuitry contained in a relatively large substrate capable of mounting large numbers of integrated circuit devices.
Such multi-layer ceramic substrates require a relatively complex metallurgy on the topside to make connections to the integrated circuit devices and provide engineering change pads, and on the bottom to make connection to the I/O pads or other type connections. When green ceramic is sintered, there is normally a 17 to 20 percent shrinkage. Frequently, the shrinkage is not uniform throughout the substrate. Since the substrate is relatively large, and the metallurgy geometry quite small, it is difficult and frequently impossible to produce a mask that is 17 to 20 percent smaller than the original substrate that will have all areas that coincide with the metallurgy placed on the substrate before sintering. Such a mask is necessary for depositing additional metallurgy layers using conventional masking techniques. Usually the original metallurgy on the substrate is deposited prior to sintering and consists of a refractory metal paste screened on the surfaces. After sintering the refractory metal must be covered with different metals, as for example, nickel, chromium, gold, etc., in order to conveniently make connections, as by solder, etc. to S.C. devices, and also to thermocompression bonded wires, and I/O pins. Frequently, these screened refractory metal layers can be covered with thin metallurgy layers by electroless plating techniques which does not require a mask. However, such coatings are usually thin and may contain impurities such as phosphorous which may be objectionable in subsequent joining operations. Metal layers can be deposited by electroplating techniques. However, an electrical connection is necessary to each area to be plated. These connections are not always available since pads to be covered can be "floating". Thus there is a need for a technique for applying relatively thick layers of metal over existing metallurgies supported on dielectric substrates, particularly sintered ceramic substrates, which does not require the formation of a mask and the usual alignment to the metallurgy pattern on the substrate.